1. Field of the Invention
2. Description of the Related Art
The invention relates to a method of laying a pipeline on a seabed and to a member for assisting in the laying of pipelines on a seabed.
A traditional approach to laying pipelines at sea is a technique known as “S-laying”. This technique involves laying pipeline from a vessel with the pipeline inclined by a small inclination such as 5 or 10 degrees to the horizontal as it leaves the vessel.
A more recent approach involves a technique known as “J-laying”, in which the pipeline leaves the vessel at a much steeper angle which may even be vertical. This method is especially suitable for laying a pipeline in deep water.
A problem associated with J-laying is the means by which additional pipe sections are added to the pipeline. Because of the steep angle of the pipeline at the vessel, the length of pipeline above sea level that can be worked on at any one time is much more restricted than in S-laying and therefore the scope for having several working stations along the length of the pipeline for working on connections of pipe sections, as is commonplace in S-laying, is much reduced.
Further problems are encountered when the pipeline in question uses what is commonly referred to as a “pipe-in-pipe” design. This design uses an inner pipe and an outer pipe. The inner pipe is used to carry fluid along the pipeline; insulation is provided around the inner pipe and the outer pipe surrounds the inner pipe and is provided to contain and protect the insulation. This configuration is commonly used to carry hot well fluids along a pipeline.
The lengths of pipeline are usually connected by butt welding. Sometimes a backing ring is placed inside the pipe at the butt joint to improve the welded joint that is obtained. The use of a pipe-in-pipe design complicates this welding process because the inner pipe must be welded before the outer pipe can be welded, so that two separate welding operations are required at one joint. Thus the time taken to complete a welded joint becomes more critical.